Circuit arrangement for producing a sawtooth current through a line deflection coil in an image display apparatus

ABSTRACT

A circuit arrangement which is the combination of a switched supply circuit and a line deflection circuit, the supply transformer being replaced by a coil.

The invention relates to a circuit arrangement for producing a sawtoothcurrent through a line deflection coil in an image display apparatus,which coil is part of a resonance circuit comprising also a tracecapacitor and a retrace capacitor, switching means applying the voltageacross the trace capacitor to the deflection coil at line frequentlyduring the trace interval of the sawtooth current, which switching meanscomprise a first diode and a controlled switch connected in parallelwith the first diode via a second diode, and an inductive element havinga winding which is connected to the resonance circuit via a third diode,the winding and the third diode passing current during the cut-offperiod of the switch.

Such a circuit arrangement is described in "IEEE Transactions onBroadcast and Television Receivers", Aug. 1972, Volume BTR-18, No. 3,pages 177 to 182. In the circuit arrangement described the said windingis the secondary winding of a transformer the primary of which has thesame number of turns and is connected between a supply source and thejunction point of the controlled switch and the second diode. During thetime in which the switch, which is a transistor, is conducting a currentflows in the primary winding so that energy is derived from the supplysource and stored in the transformer. This takes place during part ofthe trace interval. During the retrace interval, immediately thereuponand during the succeeding part of the trace interval the third diode isconducting, the said energy causing a current to flow through thesecondary winding. Because in the known circuit arrangement the end ofthe secondary winding not connected to the third diode and the retracecapacitor is connected to earth, the voltage difference between the saidend and that end of the primary which is connected to the transistor andthe second diode always is equal to the voltage of the supply source.For this purpose it is required that the transformation ratio of thetransformer should exactly be unity.

Another condition to be satisfied is that the transformer has a very lowleakage self-induction, for in the presence of leakage self-inductionthe circuit arrangement behaves as if a self-induction should beconnected in series with the primary winding. At the beginning of theretrace interval the transistor and the second diode are cut off whilstthe third diode becomes conducting. Owing to the additionalself-induction which the leakage self-induction represents, however, thesecondary current cannot immediately take over from the primary current.Between the collector of the transistor and earth a voltage peak isproduced which may be very steep and even steeper than is permissible.Hence the switch-off dissipation also will be comparatively high. As aresult the transistor may be damaged.

It is true that winding methods are known by which the impedance of theleakage self-induction can considerably be reduced, however, it cannotbe made negligible, especially at the comparatively high line repetitionfrequency, whilst the use of the said methods causes the transformer tobecome expensive. It is an object of the present invention to provide acircuit arrangement the effect of which is not different from the knownarrangement but in which the said current takeover is not performed by atransformer, and for this purpose the circuit arrangement according tothe invention is characterized in that a current which is supplied tothe controlled switch by a direct-voltage source connected between thewinding and the switch also flows through the winding.

It should be noted that in the circuit arrangement according to theinvention the said winding may still be a winding of a transformer inthe same manner as in the known circuit arrangement, for on the samecore further windings may be wound across which voltages are set upwhich after rectification may be used to supply other parts of the imagedisplay apparatus. Although the resulting transformer has leakage, thisdoes not influence the current take-over between the transistor and thethird diode.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows the known circuit arrangement,

FIG. 2 shows a modification thereof, and

FIGS. 3 to 6 show embodiments of the circuit arrangement according tothe invention.

FIG. 1 shows the circuit arrangement described in the paper referred toat the beginning of this specification. Reference symbol L_(y) denotesthe line deflection coil which is connected in series with a tracecapacitor C₁. A diode D₁ with the polarity shown and a retrace capacitorC₂ are connected in parallel with the said series combination. Thecapacitor C₂ may alternatively be connected in parallel with thedeflection coil L_(y). A secondary winding L₂ of a transformer T isconnected in series with a diode D₃ the cathode of which is connected tothe junction point of elements D₁, C₂ and L_(y) and to the anode of adiode D₂. The cathode of the diode D₂ is connected to an end of aprimary winding L₁ of the transformer T and to the collector of an n-p-ntransistor Tr. The positive terminal of a direct-voltage source V_(n) isconnected to the other end of the winding L₁ and its negative terminalis connected to the emitter of the transistor Tr. This negative terminalis also connected to the free ends of the elements L₂, D₁, C₂ and C₁ andmay be connected to earth. A driver circuit Dr controls the base of thetransistor Tr. The windings L₁ and L₂ have equal numbers of turns andare wound so that an increase of the alternating voltage at the junctionpoint of the winding L₂ and the diode D₃ corresponds to an increase ofthe alternating voltage at the collector of the transistor Tr, which isindicated by dots in FIG. 1.

During a first part of the line trace interval the diode D₁ isconducting. The voltage across the capacitor C₁ is applied to thedeflection coil L_(y) which passes a sawtooth deflection current. At agiven instant the transistor Tr becomes conducting. When about at themiddle of the trace interval the deflection current changes direction,the diode D₁ is cut off and the diode D₂ becomes conducting, so that thedeflection current now flows through the transistor Tr whilst the diodeD₃ is cut off. At the end of the trace interval the transistor Tr is cutoff. An oscillation is produced across the capacitor C₂, whilst theenergy derived from the source V_(B) and stored in the winding L₁ causesa current to flow through the diode D₃. When the voltage across thecapacitor C₂ becomes zero again, the diode D₁ becomes conducting: thisis the beginning of a new trace interval. The diode D₃ continuesconducting until the transistor, T_(r) is rendered conducting, theenergy stored in the winding L₂ being transferred to the winding L₁.Stabilisation is provided, for example, by feedback of the voltageacross the capacitor C₁ to the driver circuit Dr in which a comparisonstage and a modulator ensure that the conduction period of thetransistor Tr is varied in a manner such that the said voltage and hencethe amplitude of the deflection current remain constant.

In the known circuit arrangement shown in FIG. 1 the voltage differencebetween the ends of the windings L₁ and L₂ indicated by dots invariablyis equal to the voltage of the source V_(B), for the other ends areconnected to the positive and negative terminals respectively whilst thealternating voltages across the windings are equal. Because the sourceV_(B) and the winding L₁ are connected in series, they may in theory beinterchanged. This is the case in FIG. 2. In FIG. 2 the dot-denoted endsof the windings L₁ and L₂ always have the same potential, i.e. thewindings are effectively connected in parallel. Consequently the circuitarrangement of FIG. 2 may be replaced by that of FIG. 3 in which asingle coil L' is substituted for the two windings. Thus the transformerT has entirely been dispensed with so that the aforementioneddisadvantages cannot occur, whilst the take-over from the currentflowing through the winding L₁ by the current flowing through thewinding L₂ and vice versa cannot give rise to problems, since bothwindings have been replaced by one coil L'. The operation of the circuitarrangement of FIG. 3 obviously is identical with that of the circuitarrangement of FIG. 1. In particular, the primary current of FIG. 1flows through the loop constituted by the coil L', the source V_(B) andthe transistor Tr, whilst the secondary current of FIG. 1 flows throughthe coil L' and the diode D₃ in FIG. 3. In the coil L' the two currentshave the same direction. For the sake of clarity the driver circuit Drand the feedback path are not shown in FIG. 3.

In the circuit arrangement shown in FIG. 3 one of the terminals, forexample the negative terminal, of the source V_(B) may be connected toearth. This has the disadvantage that the emitter of the transmitter Tris not connected to a reference voltage. Retrace pulses are producedbetween the emitter and earth, which requires good insulation betweenthe secondary and primary windings of a driver transformer T' by whichthe driver circuit Dr and the base of the transistor Tr are connected.Otherwise the said pulses may interfere with the operation of thecircuit on the primary side of the transformer T'. As a result thedriver transformer T' in turn becomes more expensive. This disadvantageis obviated by the modified embodiment shown in FIG. 4 in which a p-n-ptransistor is used. In this circuit arrangement the emitter of thetransistor Tr is connected to earth for alternating currents eitherdirectly or via the source V_(B).

FIG. 5 shows another embodiment of the circuit arrangement according tothe invention. In this embodiment the transistor Tr of FIG. 4 isreplaced by a transistor of the n-p-n type, the pass direction of thethree diodes and the polarity of the source V_(B) being reversed. Inthis embodiment both the negative terminal of the source V_(B) and theemitter of the transistor Tr may be connected to earth, the effect beingequal to that of the known circuit arrangement, for at the beginning ofthe trace interval the deflection current flows through the diode D₁ andthe deflection coil L_(y) to the capacitor C₁. In the other directionthe current flows from the capacitor C₁ through the deflection coilL_(y), the transistor Tr and the diode D₂. The primary current of theknown circuit arrangement flows from the source V_(B) through the coilL' and the transistor Tr, whilst the secondary current flows through thediode D₃, the coil L' and either through the diode D₁ or to the circuitC₂, L_(y), C₁. Both currents traverse the coil L' in the same direction,so that the current take-over provides no difficulty.

In the embodiment shown in FIG. 3 the supply source V_(B) may bereplaced by a capacitor the charge of which is replenished by adirect-voltage source via choke coils. However, owing to the provisionof the coil L' one of these coils may be dispensed with. Thus theembodiment of FIG. 6 is obtained in which C_(B) is the supply capacitorand L_(B) is the choke coil connected between the source V_(B) and thecapacitor C_(B). This addition of a choke coil and a capacitor enablethe emitter of the transistor Tr of the embodiment of FIG. 3 to beearthed. It should be noted that part of the primary current and part ofthe secondary current flow via the capacitor C_(B) through the chokecoil L_(B), depending upon the value of the inductance of the coil L_(B)relative to that of the coil L'.

Similarly to the known circuit arrangement, in the circuit arrangementsshown in FIGS. 3 to 6 the coil L' may be a winding of a transformer forproducing supply voltages, whilst a winding of a high-tensiontransformer may be connected in parallel with the deflection coil L_(y).It will be appreciated that the operation of the circuit arrangementaccording to the invention is not essentially influenced thereby.

It should be noted that the known part of the circuit arrangement, i.e.that comprising the elements D₁, C₁, C₂ and L_(y), is shown veryschematically only in the Figures. Other configurations are known inwhich for example the capacitor C₁ also ensures the S-correction of thedeflection current whilst linearity-correction and centering circuitsare provided. Furthermore, the said elements may be coupled by atransformer.

What is claimed is:
 1. Circuit arrangement for producing a sawtoothcurrent through a line deflection coil, which coil is part of aresonance circuit comprising also a trace capacitor and a retracecapacitor, switching means applying the voltage across the tracecapacitor to the deflection coil at line frequency during the traceinterval of the sawtooth current, which switching means comprise a firstdiode and a controlled switch connected in parallel with the said diodevia a second diode, and an inductive element having a winding which isconnected to the resonance circuit via a third diode, current flowingthrough the winding and the third diode during the cutoff period of theswitch, characterized in that a current which is supplied to thecontrolled switch by a direct-voltage source connected between thewinding and the switch also flows through the winding.
 2. Circuitarrangement as claimed in claim 1, characterized in that both the seriescombination of the second and third diodes, which are connected with thesame pass direction, and the series combination of the winding and thecollector-emitter path of the controlled switch, which takes the form ofa transistor, are connected in parallel with the direct-voltage source,the first diode being connected between the junction point of the secondand third diodes and the junction point of the winding and thetransistor.
 3. Circuit arrangement as claimed in claim 1, characterizedin that the direct-voltage source is in the form of a capacitor, afurther direct-voltage source being connected via a choke coil to thecontrolled switch. .Iadd.
 4. Circuit arrangement for producing asawtooth current through a coil, which coil is part of a resonancecircuit comprising also a first capacitor and a tuning capacitor,switching means applying the voltage across the first capacitor to thecoil at line frequency during the trace interval of the sawtoothcurrent, which switching means comprise a first diode and a controlledswitch connected in parallel with the said diode via a second diode, andan inductive element having a winding which is connected to theresonance circuit via a third diode, current flowing through the windingand the third diode during the cutoff period of the switch,characterized in that a current which is supplied to the controlledswitch by a direct voltage source connected between the winding and theswitch also flows through the winding. .Iaddend. .Iadd.
 5. Circuitarrangement as claimed in claim 4, wherein both the series combinationof the second and third diodes, which are connected with the same passdirection, and the series combination of the winding and thecollector-emitter path of the controlled switch, which comprises atransistor, are connected in parallel with the direct-voltage source,the first diode being connected between the junction point of the secondand third diodes and the junction point of the winding and thetransistor. .Iaddend. .Iadd.
 6. Circuit arrangement as claimed in claim4, wherein the direct-voltage source comprises a capacitor, a furtherdirect-voltage source being connected via a choke coil to the controlledswitch. .Iaddend.